1.) Field of the Invention
This invention relates to a hydraulic circuit for a hydraulic shovel, and specifically to a hydraulic circuit for a hydraulic shovel, said hydraulic circuit having a plurality of hydraulic sources and being provided with a circuit for replenishing pressure oil from a predetermined one of the hydraulic sources to a predetermined actuator drive circuit when plural actuators are operated at the same time.
2.) Description of the Related Art
A hydraulic shovel carries working equipment for performing work such as excavation. This working equipment is composed of working members, such as a boom, arm and bucket, pivotally connected to corresponding pins and hydraulic actuators, such as hydraulic cylinders, for driving these working members, respectively. In actual work by the hydraulic shovel such as excavating, unloading or grading work, the working members such as the boom, arm and bucket are often operated simultaneously.
A hydraulic circuit for permitting smooth movements of working members, such as a boom, arm and bucket, in combination upon such simultaneous operation is disclosed, for example, in Japanese Patent Publication (Kokoku) No. HEI 2-16416.
The construction of an essential part of the hydraulic circuit according to this conventional art is illustrated in FIG. 8. The conventional art will hereinafter be described with reference to FIG. 8. As is depicted in the diagram, the hydraulic circuit according to this conventional art has a fist hydraulic pump 15, a second hydraulic pump 18, a directional control valve 16 for controlling a flow of pressure oil delivered from the first hydraulic pump 15, a hydraulic cylinder 14 for driving a boom 13, a directional control valve 19 for controlling a flow of pressure oil delivered from the second hydraulic pump 18, and a hydraulic cylinder 12 for driving an arm 11. On a downstream side of the directional control valve 16, a merging directional control valve 17 is arranged to guide the pressure oil from the first hydraulic pump 15 to the hydraulic arm cylinder 12. This merging directional control valve 17 is designed so that the pressure oil from the first hydraulic pump 15 is guided to the hydraulic arm cylinder 12 only when the hydraulic arm cylinder 12 is operated in an extending direction, namely, to move the arm 11 in a direction C (hereinafter referred to as the "arm-crowding direction"). Further, a by-pass circuit 41 is arranged to by-pass the pressure oil from an upstream side of the directional control valve 16 to a pressure oil feeding side of the merging directional control valve 17 via a restrictor 40. The directional control valve 16 is fed with a pilot pressure from a boom operating unit 21 and the directional control valve 19 and the merging directional control valve 17 are each fed with a pilot pressure from an arm operating unit 22, whereby the spool positions of the individual directional control valves are controlled.
According to the conventional hydraulic circuit constructed as described above, a spool of the directional control valve 16 is moved corresponding to a quantity of operation (which may hereinafter be called a "stroke") of the boom operating unit 21 so that the pressure oil from the first hydraulic pump 15 is fed to the hydraulic boom cylinder 14. When the arm operating unit 22 is operated, on the other hand, a spool of the directional control valve 19 and that of the merging directional control valve 17 are both moved corresponding to a stroke of the arm operating unit 22. When operated in the arm-crowding direction, the hydraulic arm cylinder 12 is also fed with the pressure oil from the first hydraulic pump 15 in addition to the pressure oil fed from the second hydraulic pump 18. Namely, when the boom operating unit 21 is not operated, the pressure oil from the first hydraulic pump 15 is guided to the merging directional control valve 17 through a center by-pass line 42 of the directional control valve 16. When the directional control valve 16 is operated, on the other hand, a portion of the pressure oil fed from the first hydraulic pump 15 is guided to the merging directional control valve 17 through the by-pass circuit 41 by way of the restrictor 40.
Accordingly, even upon combined operation of the boom 13 and the arm 11, especially upon operating the arm 11 in the crowding direction, the portion of the pressure oil fed from the first hydraulic pump 15 is guided, in addition to the pressure oil fed from the second hydraulic pump 18, to the hydraulic arm cylinder 12 so that the moving speed of the arm 11 can be increased.
Further, owing to the arrangement of the restrictor 40 in the by-pass circuit 41, it is possible to prevent the pressure oil from excessively flowing to a side of the hydraulic arm cylinder 12 from the first hydraulic pump 15. This makes it possible to prevent the moving speed of the boom 13 from being lowered.
As has been described above, the conventional art can improve the moving speed of the arm 11 without extremely lowering the moving speed of the boom 13 upon combined operation of the boom 13 and the arm 11. In the case of a hydraulic shovel, a higher arm-crowding speed generally leads to improvements in the operability and working efficiency upon excavation. The hydraulic circuit according to this conventional art is therefore an effective hydraulic circuit for a hydraulic shovel.
Incidentally, in excavating work by the hydraulic shovel, the boom 13 is often operated up or down while causing the arm 11 and a bucket 20 to pivot toward an unillustrated main body of the hydraulic shovel. At this time, a quantity of operation of the boom 13 is generally smaller than quantities of operation of the arm 11 and the bucket 20. In such excavating work, a flow rate of the pressure oil required for the hydraulic boom cylinder 14 becomes lower than that required for the hydraulic arm cylinder 12. Conversely, it is necessary to feed the pressure oil at an increased flow rate to the hydraulic arm cylinder 12, because the arm 11 is required to move faster and large excavational reaction force, hence, high load is exerted on the arm 11.
Upon performing such work, the above-described conventional art is unable to feed the pressure oil at a higher rate toward the hydraulic arm cylinder 12 because the restrictor 40 is arranged with its opening fixed. As a consequence, it is impossible to increase the speed of the arm 11. There is accordingly further room for improvements in the operability and working efficiency upon excavation.
In such excavating work, the quantity of operation of the boom 11 is small as described above. Large restriction resistance is therefore produced at the directional control valve 16 so that the pressure oil retained on an upstream side of the restrictor 40 is discharged into a tank through an unillustrated relieve valve. The conventional art is hence accompanied by a problem that an energy loss is large and the fuel consumption is deteriorated.